Chemical Mechanical Polishing (CMP) is the process of smoothing and/or planarizing a substrate surface by chemical and mechanical forces. The basic CMP process consists of moving a substrate surface to be polished against a pad that is used to provide support against the substrate surface and to carry a slurry between the sample surface and pad to affect the polishing. The slurry may provide both chemical action through the solution chemistry and mechanical action through an abrasive. Abrasive particles in a slurry cause mechanical damage on the sample surface, loosening the material for enhanced chemical attack by the solution chemistry or fracturing off pieces of the substrate surface into a slurry where they dissolve or are swept away.
The choice of polishing dispersion or slurry is an important factor in the CMP process. CMP is used on a wide variety of substrate materials and these different substrates have different chemical and mechanical requirements for polishing. For example, mechanical effects are important, for hard materials or non-reactive like Pt and Ta. For soft materials like Cu and polymers, chemical effects may be more important. Furthermore, a polycrystalline film may require different chemistry compared to amorphous or single crystalline films due to enhanced chemical action at the grain boundaries or at a given grain orientation
High polishing rates, planarity, selectivity, uniformity, post-CMP ease of cleaning, shelf life, and dispersion ability are factors considered to optimize the polishing composition performance. These factors are affected by many variables in the CMP process, including the abrasive particles used in the CMP slurry or in the polishing pad, which play an important role in transferring mechanical energy to the surface being polished. In current CMP dispersions, the ability to get the right mix of particle hardiness, surface chemistry, size, morphology, etc. is difficult to obtain.
Commonly used abrasive particles include silica, alumina, and cerium oxides. Each of these abrasive particles exhibits specific advantageous and disadvantageous characteristics for use in a CMP slurry. For example, cerium oxides have the advantage of being chemically reactive; therefore, it contributes to both the chemical and mechanical forces of the slurry. However, cerium oxide particles are dense, which makes it difficult to make colloidally stable. Apart from their specific characteristics, each of these commonly used abrasive particles is limited in the size and morphology achievable for the particle when it is used alone in the CMP slurry.
Additionally, other materials, such as carbon black, are being investigated as abrasives for CMP slurries. Carbon black molecules have some advantageous characteristics for use in CMP, such as being chemically neutral, which allows it to be used on a variety of substrates and in a wide variety of chemical environments. However, carbon black also has dispersability and stability disadvantages. Additionally, carbon black tends to adhere to surfaces being polished and is difficult to clean off of the polished surfaces.
Therefore, there is a need for abrasive particles that can be specifically designed to maximize the specific characteristics needed for CMP processes. There is a need for abrasive particles that can be specifically designed to optimize the slurry performance for the specific CMP process in which they are utilized.